Field help (DEFCTMBPS2) 

Select this check box if this supplier is a default back-to-back order supplier for this product at this site.

When generating a back-to-back purchase order and during bulk generation of back-to-back orders, the supplier priority level is cross-referenced with the blocking level to determine the default back-to-back order supplier for this product.

A back-to-back order supplier that has received a warning can still be the default back-to-back order supplier if it has the highest priority level.

For the transactions: